1. Field of the Invention
The present invention involves a method and apparatus for depositing a silicon oxide onto a substrate. More particularly, the present invention involves a method and apparatus for depositing a silicon oxide onto a substrate from solution at low temperatures in a manner that produces homogeneous growth of the silicon oxide.
2. Description of Related Art
There are various methods used to grow inorganic materials onto a substrate that are well-known in industry. However, these existing methods all have significant negative attributes that add to the expense and complexity of the process. For example, the growth of thin films of inorganic material in the electronics industry is often accomplished through the use of chemical vapor deposition (CVD). CVD requires the pyrolysis or photolysis of volatile compounds to create chemical fragments that are deposited on the surface of a substrate. The temperature of the substrate must be sufficiently high to allow mobility of fragments on the growth surface. These fragments travel around the surface until they find thermodynamically stable sites to which they attach. CVD therefore requires the use of high temperatures, volatile compounds or low pressures, each of which adds to the environmental load of the process.
Another method used to grow inorganic materials is sol-gel. Although sol-gel is a low temperature method, it involves multiple steps to form a true inorganic material. In sol-gel, precursor compounds are dissolved in solution and reacted with additional reagents (usually water or an acid) to give a gel. If a film or coating is required, then the gel must be spin-coated onto the substrate. Since most sol-gels consist of nanoparticles or clusters with a significant organic content, additional thermal or chemical treatments are required to form a true inorganic material.
Previous research (U.S. Pat. Nos. 2,505,629; 5,073,408; 5,132,140) has described processes for deposition of SiO2 layers on silicon surfaces using a room temperature (30 to 50° C.) solution growth. The growth of liquid-phase deposited (LPD) SiO2, for deposition of SiO2 on the surface of soda lime silicate glass, is based on the chemical reaction of H2SiF6 with water to form hydrofluoric acid and solid SiO2. However, one of the major disadvantages of previous SiO2 LPD methods is the very low deposition rate of about 8 nm/hour. This low deposition rate makes it impractical for growing insulator layers for most applications. There exists, therefore, a need for a method to uniformly grow inorganic materials in a low temperature process with a high deposition rate.
In order to increase the rate of film growth several additives have been used. Initially most researchers investigated boric acid (U.S. Pat. No. 4,468,420; Nagayama, et al., 1988; Homma, et al., 1993; Chou and Lee, 1994; Huang, et al., 1998). As an alternative various metal compounds have been investigated (U.S. Pat. Nos. 4,431,683; 4,693,916) or a combination of organic additives (U.S. Pat. No. 6,080,683). However, while these approaches do result in faster film growth, the additives are incorporated into the films and in some cases film quality suffers from the presence of the additives. Further approaches involve the use of photolysis to assist film growth (Huang, et al., 1996) or electrolytic reactions (U.S. Pat. No. 5,616,233).